test: lock parser span-retention behavior

tests/NATS.Server.Core.Tests/Protocol/ProtocolParserSnippetGapParityTests.cs

@@ -42,4 +42,15 @@ public class ProtocolParserSnippetGapParityTests
         ex.Message.ShouldContain("Maximum control line exceeded");
         ex.Message.ShouldContain("snip=");
     }
+
+    [Fact]
+    public void Parse_invalid_payload_trailer_preserves_existing_error_message()
+    {
+        var parser = new NatsParser();
+        var input = Encoding.ASCII.GetBytes("PUB foo 5\r\nHelloXX");
+        ReadOnlySequence<byte> buffer = new(input);
+
+        var ex = Should.Throw<ProtocolViolationException>(() => parser.TryParse(ref buffer, out _));
+        ex.Message.ShouldBe("Expected \\r\\n after payload");
+    }
 }